1. Field of the Invention
This invention relates to injection moldable and extrusion type electrically conductive polymeric blend compositions having a viscosity at 200.degree. C. at 0.73 sec.sup.-1 of less than about 8.times.10.sup.5 poises. The compositions used for elastomeric articles include a neutralized sulfonated polymer; graphite fiber at a concentration of about 1 to about 50 parts by weight per 100 parts by weight of neutralized sulfonated polymer; and a preferential plasticizer at less than about 60 parts by weight based on 100 parts by weight of the neutralized sulfonated polymer. The composition may also optionally include fillers, oils and other additives. These blend compositions can be readily processed due to their superior rheological properties on conventional plastic fabrication equipment, especially on low and high pressure injection molding or extrusion equipment into electrically conductive articles having excellent physical and electrical properties.
2. Description of the Prior Art
Although polymers are in general non-conductive and are used in a multitude of insulating applications, there is a considerable commercial interest in conductive rubbers and plastics. For example, conductive compounds have been used for the prevention of corona discharge in cables, for antistatic purposes, and for heating. Antistatic polymers can be used in situations where explosive vapors, liquids or powders are handled, for aircraft tires in order to conduct to the ground the static charges generated by the tires or acquired in flight, or for manufacturing equipment in order to dissipate the static charges generated on production line machinery (for example, rolls and conveyor belts). Other applications for conductive rubbers include materials for heating purposes or deformable circuit components such as a conductive keyboard for a typewriter or a calculator.
Conductive rubbers have an advantage over metals in that they are flexible and are able to absorb mechanical shock. Furthermore, they are easily formed into complex geometrical shapes, have low densities, can be compounded to yield a wide range of electrical conductivities and thermal conductivities, and can be more corrosion resistant than metals. Disadvantages of conventional conductive rubbers versus metals are that they generally have lower mechanical properties, have a more restrictive useful temperature range, and cannot be produced with conductivities as high as metals.
The traditional conductive rubber consists of an elastomer filled with carbon black, graphite powder, or metal powders. In general, in order to achieve a high conductivity in particulate filled elastomers, a relatively high loading (at least 20 weight percent) is required. For example, Gurland (Trans. Met. Soc. AIME, 236, 642 (1966)) has shown that plastic compounds become electrically conductive with the addition of 38 volume percent conductive spheres randomly distributed in the polymer. Malliaris and Turner (J. Appl. Phys., 42, 614 (1971)) were able to make conductive polymers with 6 volume percent metal particles, but were unable to achieve this on a large scale. Norman (Conductive Rubbers and Plastics, Elsevier Publ. Co., New York, 1970) gives relationships between electrical resistivity and carbon black loadings for numerous filled rubbers and different carbon blacks. In all cases, loadings greater than 20 phr by weight are needed in order to achieve a low resistivity (i.e., of the order of 10.sup.2 -10.sup.4 ohms-cm).
The instant invention relates to electrically conductive polymeric blend compositions which include carbon or graphite fibers blended in a neutralized sulfonated polymer. Graphite fibers have been used to prepare electrically conductive elastomeric (rubber) compositions. For example, Japan Kokai No. 77-59,645 described a composition based on 2-80 phr by weight of graphite fibers in a vulcanized styrene butadiene rubber. While such materials may be of value they require the time consuming and expensive step of vulcanization, and once that step is effected, the resulting products are not reprocessable. In contrast, the polymeric compositions of the instant invention can be readily processed on conventional thermoplastic machinery, i.e., extrusion or injection molding. Further the inventive compositions possess the strength and rigidity normally associated with cured rubbers. The ability to process these materials as thermoplastics means that these materials can be molded economically into a variety of intricate shapes and can be reprocessed readily. The electrically conductive polymeric blend compositions of the instant invention can be formed from carbon or graphite fibers at significantly lower levels of incorporation of carbon or graphite fibers than levels needed for carbon black while maintaining excellent physical properties at these lower levels of graphite fiber loading. The data given in the examples of the instant invention clearly demonstrate that electrically conductive polymeric blend compositions can be prepared using graphite fibers at loadings significantly lower than for carbon black. A range of conductivities (conversely, resistivities) can be achieved by varying the fiber loading and/or the fiber length. These polymeric compositions can be processed on typical polymer processing equipment such as a Brabender or Banbury mixer, a continuous intensive mixer, single and twin screw extruders, and injection molding equipment. The data of the instant invention further demonstrates that the polymeric blend composition can be made from a blend of a plasticized sulfonated polymer with graphite fiber wherein the polymeric blend composition is formed into the conductive fibers, pads, or other complex shapes. These polymeric blend compositions of sulfonated elastomers or sulfonated plastics can be processed by conventional thermoplastic techniques or cast from solution.